1. Field of the Invention
The present invention relates to capture gamma ray spectroscopy measurements of earth formations traversed by a borehole and, more particularly, to improved methods and apparatus for correcting such measurements for the effects thereon of borehole geometry and constituents.
2. The Prior Art
The measurement of gamma ray energy spectra produced by the capture of thermal-energy neutrons has been shown to be a useful technique to determine the elemental composition of earth formations. To realize the full potential of these measurements, three separate problems must be addressed: (1) each measured gamma ray spectrum must be decomposed into contributions due to individual atomic elements or "elemental yields", (2) the contributions of each elemental yield due to formation elements must be separated from the contributions due to elements within the borehole, and (3) important petrophysical parameters such as porosity, matrix lithology, and water salinity must be derived from the formation elemental yields.
Although the first and third of these problems have been extensively addressed in the prior art, see, for example, U.S. Pat. Nos. 3,521,064 and 4,055,763, the second problem has often been either ignored, treated as a minor perturbation, or treated too simplistically. In fact, however, the measurement sensitivity to borehole constitutents can easily be as large as or even larger than the sensitivity to formation constituents. Moreover, even for a constant borehole geometry, this relative borehole contribution is variable from depth to depth within the borehole depending upon such parameters as borehole and formation salinities and formation porosity. An effect such as this, which is both large and variable, can neither be ignored nor handled in general by a down-hole calibration as has heretofore been the case. One approach to this problem that has been used commercially has been to divide the borehole/formation environment into different regions and to estimate the geometrical effects of each region on the borehole contribution to the detected gamma ray spectrum. Although affording improved results relative to earlier prior art techniques, this approach does not provide a full solution to the problem inasmuch as it deals only with geometrical effects and does not take into account the influence of such variables as fluid salinity and porosity.